1. Field of the Invention
The present invention lies in the field of medical technology and generally relates to a device for repairing cartilage defects and/or cartilage/bone defects in human or animal joints. More specifically, the device serves to repair defects in the cartilage layer, which in joints covers the bone surface, or of defects that concern this articular cartilage layer and also bone tissue lying thereunder.
2. Description of Related Art
Damage to articular cartilage by way of injuries or involution caused by aging or disease is particularly common in humans. Very often such damage also takes its toll on the bone tissue lying below the articular cartilage. The degree of damage to articular cartilage defects and/or cartilage/bone defects is determined with the help of the Outerbridge scale, with the following categories: superficial fraying (approx. 10% of all cases), cartilage fissure (approx. 28%), fissure down to the bone (approx. 41%), damage involving cartilage and bone (approx. 19%) and other damage such as osteochondritis dissecans and joint fracture (approx. 2% of all detected cases).
Vital cartilage tissue contains living cells by way of whose activity the specific intercellular cartilage matrix is built up during adolescence. However, it contains very little vascularization in the fully grown condition and, therefore, has a very limited regeneration capability. This means that cartilage defects or cartilage bone defects, in particular those defects concerning a relatively large cartilage surface, do not heal by themselves and therefore must be repaired by surgery (Mankin H J: The response of articular cartilage to mechanical injury, Journal of Bone and Joint Surgery (Am) 64A (1982) March: pages 460-466).
For repairing the named defects it is, for example, suggested to implant devices comprising the tissue to be repaired or a perform of this tissue. Such devices are cylindrical and comprise a cartilage layer on one end face. For implantation a pocket-hole shaped opening or bore is produced in the region of the defect to be repaired and the device is positioned in the bore such that the cartilage layer of the implant faces towards the outside. The bore, independently of the depth of the defect, extends into the healthy bone tissue. The device has a somewhat larger diameter than the bore and the same axial length. Therefore, after implantation there is a radial tension (press fit) between native tissue and the implanted device by way of which the implant is held in the bore. The cartilage surface of the implant is flush with the surrounding native cartilage surface. The devices have, according to the size of the defect, a diameter of 4 to 10 mm (e.g. 5.4 mm for the device and 5.3 mm for the bore) and lengths of approx. 10 to 20 mm.
For larger defects it is suggested to implant a plurality of such cylindrical devices in the defect region in a mosaic manner and to fill out the intermediate spaces between the implants with a suitable material.
The cylindrical devices are for example autologous (auto-transplants). For the repair of an articular cartilage defect concerning a heavily loaded location of a joint, a suitable tissue piece is harvested, for example, from a less loaded location of the same joint and is transplanted into a bore created at the defect location using a hollow drill (Hangody L et al.: Mosaicplasty for the treatment of articular cartilage defects: application in clinical practice. Orthopedics 1998 Jul., 21(7):751-6).
The cylindrical devices may also originate from a suitable donor (homologous transplants). Also known are suitable heterologue implants or xenotransplants which before implantation are suitably treated, e.g. photo-oxidized (as described in the publication EP-0768332 of Sulzer Innotec), for preventing immune-reaction after implantation or for minimizing such immuno-reaction (immunological deactivation). Such implants are, for example, removed from shoulder joints of slaughtered cattle and have the advantage of being available in much larger numbers than autologous or homologous transplants and of causing no secondary defects on harvesting, which secondary defects must be repaired and lead to new difficulties.
In the publication WO-97/46665 (Sulzer Orthopedics) a suitable device is described of which the bone part consists of bone replacement material and the end-face cartilage layer is grown onto it in vitro from autologous chondrocytes.
In all mentioned devices being made from natural tissue there is a natural connection or coalescence between the end-face cartilage layer and the bone part and there is an outermost bone region (subchondral bone plate) in which the bone tissue is more compact than in other bone regions. The mentioned, partly artificial implants also show the coalescence of cartilage layer and bone part and the artificial bone part is advantageously equipped with a more compact, that is to say less porous, outer layer which serves the cartilage layer as an underlay.
An important function of the subchondral bone plate or an artificial imitation thereof is evidently the prevention of vascularisation of the cartilage layer proceeding from the bone tissue, which would lead to ossification of the cartilage. In addition the subchondral bone plate having a higher density than the inner bone tissue represents a region of higher mechanical strength.
With the devices as mentioned above it is attempted to achieve the following targets:                The bone part of the device is to allow solid anchoring of the implant by way of a press fit, in a manner such that the implant requires no further fastening means interfering with healthy cartilage regions.        The coalescence of cartilage layer and bone part in the device is to give the implant stability so that the cartilage layer cannot be detached and removed from the defect location, even if the joint is not immobilized after implantation.        The cartilage layer of the device Is to have a mechanical strength and elasticity such that the repair location may be fully loaded directly after implantation.        The cartilage layer is to form a zone in which conditions suitable for the implanted cells or for cells migrating into it after implantation prevail, such that the cell can produce or maintain a fully functional cartilage tissue. This is also to be supported by the subchondral bone plate which separates the cartilage layer from the bone part and which helps to prevent vascularisation proceeding from the bone part.        The bone part is to represent a zone in which conditions suitable for the implanted cells or for cells migrating into it after implantation prevail, such that they can produce or maintain a fully functional bone tissue.        
New trials in which artificially produced defects in joints of sheep have been repaired with auto-transplants, homo-transplants or with hetero-transplants (from cattle tissue) in the previously mentioned manner, show that the healing process after implantation does not proceed as expected.
In particular, it has been shown that the bone part of the implants is not integrated in the native tissue or replaced gradually by new reparative tissue, but that the bone part of the implant undergoes a transformation process with essentially three successive phases. In a first step bone osteoclastic cells (osteoclasts) are stimulated and the implanted bone starts to be resorbed. This first phase is already clearly visible six to eight weeks after implantation. A hollow space (cyst) then arises in the implant and is filled with connective tissue. This second phase reaches a climax after approximately six weeks. In the third and last phase bone-forming cells (osteoblasts) are attracted which convert the connective tissue to bone. This conversion process is concluded after about twelve months. Then the newly created bone structure is so well adapted that the original border between the implant and the surrounding bone tissue can hardly be perceived anymore.
Due to the described, three-phase transformation process comprising a middle phase in which the cartilage layer of the implant is not carried by the bone part capable to do so but by a mechanically inferior cyst, there exists a high risk that the cartilage layer is pressed into this cyst where it can neither fulfill its mechanical nor its biological function and from where it cannot be displaced during the following phases of the healing process. This risk significantly reduces the chances of healing success. Healing with a badly positioned cartilage layer causes negative after-effects.
It is surprising that the trials show the cyst formation at the location of the bone part of an implant in a middle phase of the healing process not only for homologous and heterologous implants, but in particular also for auto-transplants. The initial resorption of the implanted bone tissue does not therefore appear to be an immuno-reaction in which implanted vital material is recognized as foreign and is therefore resorbed. It would appear that it is rather a reaction to implanted, dead material. This means that by cutting off the natural blood supply on harvesting the implant even when harvesting it from viable tissue and even when it is implanted directly after harvesting, the bone tissue loses its viability. In any case, the bone part of the implant is resorbed and is rebuilt only after substantially complete resorption.